Conventionally, for use in precise filtration of gas or liquid or in the application requiring separation, such as separator of a battery, a microporous film having air permeability is often used. As a simple method for producing such a microporous film, a method where polypropylene (hereinafter referred to as PP) is kneaded with an inorganic filler particle such as calcium carbonate, silicon oxide and barium sulfate, film-formed and then stretched to generate micropores was initially known. The microporous film obtained by such a method has a problem that ash remains after burning on disposal or due to low compatibility of an inorganic filler particle with a polyolefin, the inorganic filler particle falls off as dust during production or during use.
Among the above-described applications of the microporous film, in the case of a separator for a lithium ion battery, it is required that even when the battery temperature further rises after shutdown at the time of temperature rise, the battery is safely terminated by maintaining insulation without breakage of the separator (short-circuit) until high temperatures. In order to satisfy these requirements, various methods, such as coating with a heat-resistant layer or formation of a multilayer separator structure, have been attempted. However, the production method is complicated or thinning is difficult. The task of further enhancing the dynamic physical properties and heat resistance has not yet been solved.
In the method for producing such a microporous film, a polyolefin-based resin is used. As the method for producing a microporous film by using this resin, two methods, i.e., a wet process (phase separation method) and a dry process (stretch method), are widely known.
In the wet process, a polyolefin-based resin and a plasticizer such as paraffin are melted/kneaded in an apparatus having kneading ability, such as extruder or desktop kneader, and then extruded into a sheet shape by using a T-die, etc. Phase separation occurs between the plasticizer and the resin when the resin extruded into a sheet shape is cooled on a roll, and the sheet is then biaxially stretched longitudinally and transversely either sequentially or simultaneously to thereby being thinned. Subsequently, the plasticizer in the thin film is extracted and removed using an organic solvent, etc. to obtain a microporous film. However, in this method, a plasticizer extraction step is required and therefore, there is a problem that the production process is complicated and the production line becomes long. In addition, treatment, adverse effect on the human body, etc. of the organic solvent used for extracting the plasticizer becomes a problem (Patent Documents 1 and 2).
On the other hand, as for the dry process, there is a method where a polyolefin-based resin is melted/kneaded in an apparatus having kneading ability, such as extruder or desktop kneader, and then formed into a sheet shape by using a T-die, etc. and the sheet is thinned at a high draft ratio, then subjected to heat treatment to produce a crystal of high regularity in the sheet and thereafter, stretched at low temperature and high temperature to cause crystal interfacial peeling and create an interstitial portion between lamellas, thereby forming a porous structure. In this method, unlike the above-described wet process, an extraction step is not required, and the production process can thereby be simplified, but due to uniaxial stretching, the stretch ratio is low, and the yield of the microporous film obtained is low, leading to a cost rise. In addition, the method has a problem that the strength in the width direction is low and the film is likely to be longitudinally torn when punctured by a sharp projection (Patent Documents 3 and 4).
In order to cope with this problem, a β-crystal method in which a film-like material obtained by kneading and molding a PP composition including a PP substrate in place of an inorganic filler particle and a β-crystal nucleating agent is stretched at a temperature of 120 to 150° C. to provide good air permeability has been developed (Patent Document 5). However, even by this method, heat resistance can only be maintained generally up to approximately from 160 to 170° C., though the melting point of PP varies depending on the molecular weight and stretching conditions.
Patent Document 6 describes the result that when CeNF is compounded into a polyethylene substrate by a wet process, the separator properties are greatly enhanced compared with conventional separators. This result clearly reveals the effect due to compounding of CeNF in a separator, but the substrate is a polyethylene and since the melting point thereof is approximately from 130 to 140° C., though it may vary depending on the molecular weight or stretching conditions, the heat resistance is lower than in the dry process using a PP substrate. In addition, because of use of a degreasing solvent in the wet process, the production process is complicated, leading to a high cost, and there is an environmental problem.